Electron discharge tube



Oct.26, 1937. ASEV v Y 2,097,258

ELECTRON DI SCHARGE TUBE Filed Jan. 51, 1934 I1 g.i 1 119,2

ATTORNEY CJI Patented Oct. 26, 1937 ATENT OFFICE ELECTRON DISCHARGE TUBEAlexandre Sev, Paris, France, assignor to Compagnie Generale deTelegraphic Sans Fil, a corporation of France 2 Claims.

The present invention is concerned with power amplifiers used in publicspeaking, talking films, etc., and more particularly symmetricamplifiers known as of the push-pull pattern.

It is known that the symmetric coupling of the tubes in push-pullamplifiers allows of eliminating certain distortions due to thecurvature or knee of the characteristic curve.

Based upon this principle high-power amplifiers of the so-called class Btype have been constructed in which the curved portions of thecharacteristics are employed. However, it is admitted that thedistortions due to the knee are not entirely eliminated by a push-pullscheme.

Now, the present invention has as its object a high-power amplifierwherein the distortions due to power tubes connected push-pull fashionare practically obviated. It consists in a novel mode of couplingbetween the inputtube and the power tube next above it. This couplingmethod allows of eliminating the curved portion in characteristics ofpower tubes, while maintaining the output or efficiency at a highfigure.

A further object of the invention is to provide a normal or fixed biasvoltage to the grid of an amplifier tube and means whereby positivesignal impulses also are impressed on the grid while-the negative signalimpulses are substantially prevented from reaching the grid.

A still further object is to provide a vacuum tube construction in whicha diode having an indirectly heated cathode and a triode having a heatedfilament are enclosed in the same glass envelope.

Other objects of the invention will become apparent to those skilled inthe art as the description thereof proceeds. For a better understandingthereof reference is made to the following description taken inconnection with the accompanying drawing, in which,

Figure 1 shows the curve of the plate current as a function of thepotential of the grid or control electrode in a multi-electrode tube;

Figure 2 shows the coupling of characteristics of a push-pull amplifierof class B, so-called;

Figure 3 represents an embodiment of a coupling forming the object ofthis invention as applied to a push-pull amplifier;

Figure 4 represents a different embodiment predicated upon the sameprinciple;

Figure 5 represents schematically a double tube adapted to operate incircuits as shown in Figures 3 and 4; and,

Figure 6 illustrates in a general way a mode of coupling as heredisclosed.

It will be seen that the characteristic curve of Figure 1 comprises atits lower end a marked curvature or knee. In certain applications, andmore particularly in the amplification of telephone currents, itisnecessary to suppress the effects of the curvature. Usually recourse ishad to two methods, to wit:

(1) In amplifiers belonging to what is called class A the working point(point A, Figure l) is chosen in a way so that the variations of theplate current Ip will keep inside the limits where Ip f(Vg) may beregarded as a linear function.

The efiiciency of such amplifiers (class A) cannotgo beyond percent, butin practice it is rarely ever over 25 percent.

(2) In amplifiers of class B, the working point corresponds to a lowpermanent plate current and is usually located in the curved portion-(point B, Fig. 1). To avoid distortions two power tubes are connectedsymmetrically in phase opposition according to the conventionalpush-pull scheme.

Figure 2 shows the coupling of the characteristics of two tubes and bybroken line the resultant characteristic. The efiiciency may exceed 60percent, but the compensation of the curved portions is oftenincomplete. Whence a distortion is present that will be especiallyappreciable where low powers are involved.

Now, the method forming the object of the present invention consists ininterposing between the input tube or previous stage and the grid of thepower tube a circuit comprising an electronic valve, which will permitof the flow to the power grid only of the positive alternation of thepotential wave. The working point of the power tube is shifted justabove the knee to point C, Fig. 1 by providing a suitable grid bias; andduring actual operation it will be able to shift only towards theright-hand side, i. e., into a region where characteristics are sensiblyrectilinear.

One arrangement based upon this principle is indicated in Fig. 3 whichrepresents an amplifier for telephone currents of the push-pull type. Inthe absence of applied signal voltage the valves V1 and V2 arenegatively biased by virtue of the fall of residual current in or acrossthe resistances R, the value of which is chosen so as to render thiscurrent very small or a few microamperes. When the potential V appliedto the anode of V1 is positive the variation of anode current 2' ispractically proportional to V. If R is sufiiciently high the ohmic dropacross the valve is negligible compared with V. Thus the positivealternation of V is transmitted to and impressed upon the control grid 4of L1.

When the voltage V becomes negative, the internal resistance of thevalve V1 grows very fast and the fraction of V transmitted to the grid 4is very small or negligible. The grid of L2 is fed in the same way bythe valve V2. By virtue of the symmetric disposition, the lost voltagesof the two alternations are partly compensated.

It will be seen that each of the tubes L1Lz is operative during analternation. The curved portions of the characteristics L1Lz aresuppressed while yet preserving the advantages of amplifiers of class B.As shown, the signal voltage is applied to the control grids by an audiotransformer l2 whose primary coils are connected in the plate circuitsof tubes it .and IL. The fila-' ments of tubes L1L2 and the heaters 2 ofV1 and V2 (whose cathodes 2 are indirectly heated) may be heated byalternating current supplied by a transformer IS. The plate current oftubes L1L2 may be supplied by a battery or other voltage source l4connected as shown, and their output may be connected to any translatingdevice such as a loudspeaker I5.

Another circuit scheme predicated upon the same principle is illustratedin Fig. 4. The valve V1 connected in parallel with the grid of L1 keepsit at the potential of the common point C during the negativealternation of the voltage. At the same time, the internal resistance ofV2 is infinitely high, and the positive alternation is transmitted andfed tothe grid of L2, etc.

It may be. of advantage in certain circumstances, especially whenthereverse current of the grids of L1 and L2 is high, to modify thepolarization of the valves without affecting R. For this purpose, theremay be interposed between and A (Fig. 3) a biasing battery 16 or else asuitable resistance traversed by a direct current. This current maybe'furnished from the plate circult of the input tube or from that ofthe tubes 111142 by way of two taps on resistance 1'. It will be notedthat the polarization of the power grids (Fig. 3) is obtainedautomatically by means of the resistance r shunted by a condenser I!which is the conventional arrangement in the casev of amplifiers, classA, but which when applied to the case of amplifiers class B of thenormal type is attended with serious difficulties.

It is also feasible to s'ustitute a battery for resistance 1'.

In Figures 3 and 4 the filaments of the valves L1-L2 are heated inparallel withthe heaters of the tubes V1V2, the midpoint 18 of theheating winding being united with the biasing resistor r. In the case ofthe directly heated valve it is generally necessary to reduce thecapacity benot ces tween the heating winding and ground by the use ofspecial transformers.

In the embodiments shown in Figures 3 and 4, it is possible to useeither valves with a high vacuum or else gaseous valves. It is alsopossible to substitute multi-electrode tubes for the valves, thecharacteristics of which present a suitable knee so as to insure a kindof operation as hereinbefore described.

In order to reduce size and space, according to another object of thisinvention, recourse may be had to a double type of tube comprising theelements of both valve V and power tube L. Such a tube is represented inFigure 5. The two filaments are heated in parallel, the cathode 2 ofthe'valve is connected internally with the power grid 4.- The number ofoutlets or prongs islimited to five, all the tube elements being mountedin a single envelope 20.

It is understood that the arrangements of Figures 3 and 4 are. citedmerely by way of example, for the invention'is capable of a great numberof modifications as will be evident to those trained in the art.Generally speaking, the coupling arrangement of the invention is shownin principle and in its most abstract form in Fig. 6. The circuit Pconnected to the plate of the input tube is coupled with the circuit Gconnected with the power grid by means of a circuit V comprising anelectron valve. The coupling between the three circuits is arranged in away so that the variable potential applied to grid G will not becomenegative. The working point of the power tube is thus maintained in thelinear portion. While shown with an audio amplifier, the same principlemay be used in radio frequency amplifiers comprising one tube or twotubes in a'push-pull arrangement.

Having described my invention what I claim as new and desire to secureby Letters Patent is:

1. A vacuum tube having an amplifier portion comprising a heatablefilament, a grid and a plate, and a rectifier portion comprising aheater, a cathode and a plate, a single envelope enclosing all of theabove named elements, connections within' the envelope between thefilament terminals and the heater terminals and also between the gridand the rectifier cathode.

2. An electron discharge tube comprising an evacuated envelope enclosinga filamentary cathode, a grid next adjacent thereto and an anodeconstituting an amplifier, an indirectly heated cathode and an anodeconstituting a rectifier, and a direct electrical connection within theenvelope between the indirectly heated cathode and the ALEXANDRE SEV.

